i thought i was getting a good deal with some conti's that looked like they had almost all of their tread remaining for about half the price new. until the tread started cracking 10,000KM later and i saw that the DOT date is 2017. The rubber on them is hard, but they are otherwise gripping and wearing decently. I've driven more than 30,000KM on them so far and have maybe about 5/32nds remaining on the fronts. But the cracking is kind of bad. I wouldn't do it again.
i would only buy used tires if the DOT date on them is reasonable. otherwise, you're best off just getting new ones.